翻译 改文档格式.docx
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翻译 改文档格式.docx
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学生姓名:
陆煜斌学号:
1204102001
专业:
电气工程及其自动化
所在学院:
机电学院
指导教师:
王海祥
职称:
副教授
2016年3月4日
PreventionofIslandinginGrid-connectedPhotovoltaicSystems
Recentlytherehasbeenaresurgenceofconcernaboutislandingofgrid-connectedphotovoltaic(PV)systems.ThisconditionoccurswhenthePVsystemcontinuestoenergizeasectionofthegridafterthatsectionhasbeenisolatedfromthemainutilityvoltagesource.Generally,islandingisundesirablebecauseitposesasafetyhazardtoutilityservicepersonnel,andalsobecauseitcanleadtoasynchronousreclosurewhichcandamageequipment.ItisthereforeimportantthatPVsystemsincorporatemethodstopreventislanding.Thepurposeofthispaperisthreefold:
(1)tocriticallyreviewtheliteratureonislandingpreventionmethodsforPVsystemsanddiscusstheirstrengthsandshortcomings;
(2)toreviewandanalyzetheislandingbehavioroffourconverterswhichareprominentintheliteratureinordertodemonstratetheimplementationandeffectivenessofsomeislandingpreventionmethods;
and(3)tointroduceanewislandingpreventionscheme,activefrequencydriftwithpositivefeedback,whichovercomesmanyoftheshortcomingsofexistingschemes.Itisconcludedthatno`perfect'
islandingpreventionmethodyetexists,butalsothatmanyexistingmethodsorcombinationsthereofworkverywellinpracticalsituations.Finally,itisnotedthananinvestigationofwhatconstitutes‘sufficient’islandingpreventionisneeded.
INTRODUCTION
Inmanycountriesthroughouttheworld,programsarebeingimplementedtoencouragetheinstallationofgrid-connectedphotovoltaic(PV)systems.Theproliferationofsuchsystemshasledtoaresurgenceofconcernabouttheproblemofislanding.IslandingoccurswhenaPVsystemfeedspowerintoasectionoftheutilitysystemwhichhasbeenisolatedfromtheutilityvoltagesource.APVsystemconnectedtoafeederlinewhichisinturnconnectedtotheutilitygridthroughatransformerandsomesortofswitch(arecloser,breaker,fuseetc.).ThePVsystemconsistsofaPVarrayandapowerconditioningunit(PCU).Alocalloadisalsoconnectedtothefeederline.Iftheswitchwereopened,undercertainconditionsitispossibleforthePVPCUtocontinuetoenergizetheisolatedsectionofthegridandsupplypowertothelocalload.Thisisislanding,andtheisolatedsectionoftheutilitybeingpoweredbythePVsystemisreferredtoasanislandofsupplyorsimplyanisland.Utilitiesfrequentlyusetheterm‘renewableenergyisland'
todifferentiatethissituationfromothertypesofislands.Althoughthisdistinctionisfrequentlyimportant,forbrevitywewillusetheterm`island'
throughoutthispaperwithoutambiguity.
TheamountoftimebetweenthedisconnectionoftheutilityandtheshutdownofthePCUisreferredtoastherun-ontime.Islandingeventsaretypicallysubdividedintotwocategories:
long-term,withrun-ontimesof1secormore,andshort-term,withrun-ontimesoflessthan1sec.
Theprimaryconcernwithlong-termislandingisoneofsafety.Maintenanceorrepairpersonnelarrivingtoservicetheisolatedfeedermaybeunawarethatitisstillenergized,whichcouldleadtopersonalinjury.Thisisofparticularlygreatconcerninthecaseofscheduledmaintenance,whentheswitchwouldbemanuallyoperatedbyservicepersonnelwhowillimmediatelycommenceworkontheisolatedsystem.Inthiscase,islandingofevenafewtensofsecondscouldbedangerous.
Anotherproblemassociatedwithbothlong-andshort-termislandingisthatthePVsystem,whichreliesontheutilityvoltagetoprovideaphaseandfrequencyreferenceforitsoutputcurrent,maylosesynchronismwiththeutilitywhiletheswitchisopen.TheutilitycouldthenrecloseonaPVsystemwhichisoutofphase.MostPVPCUsaretwoquadrantdevices,designedforunidirectionalpowerflowfromtheDCtoACsideonly.Thisisdoneforeconomicreasons;
two-quadrantconvertersarelessexpensivethanfour-quadrantconverters.However,duringanout-of-phasereclosure,thereareintervalsinwhichthepolaritiesofthevoltageandcurrentareopposite.Duringtheseintervals,theconverterisabsorbingpowerfromboththePVandutilitysides,whichcanleadtodestructivecomponentfailuresinthePCU.
Ithasbeenpostulatedthatanotherpossibleproblemwithshort-termislandingisthatitcaninterferewiththearc-clearingfunctionofprotectiverelays.9However,thereismuchdebateoverwhetherthisisasignificantissue.Thissubjectwillbediscussedinmoredetailshortly.
Onefinalproblemwhichisincreasinginrelevanceisthatsomeislandingpreventionmethodsinterferewitheachother,leadingtolongerrun-ontimesandpossiblyfailuretodetectislandingifseveralPVsystemsarepresentintheisland.Insomecases,thiscanhappenevenifallthePVsystemsintheislandareusingthesameislandingpreventionscheme.Thissituationhasbeentermedthe`multi-invertercase'
anditcouldbecomeincreasinglycommonwiththeproliferationofsmall,roof-mountedPVsystemsandthedevelopmentofPCUsforACPVarrays,inwhichcasetherecouldbetensorevenhundredsofPCUsinanisland.
Aconsiderablebodyofworkexistsonislandingandislandingpreventionmethods.Thepurposeofthispaperisthreefold:
first,tocollectandreviewtheexistingmethods,discusstheirstrengthsandweaknesses,andcomparethem;
second,toreviewandanalyzethebehavioroffourexistingconverterstodemonstratetheeffectivenessandimplementationofthesemethods;
andthird,toproposeanovelislandingpreventionscheme,activefrequencydriftwithpositivefeedback(AFDPF),whichlargelyovercomestheshortcomingsoftheexistingmethods.
ThereactanceinsertionmethodisuniqueamongislandingpreventionmethodsinthatitdoesnotrelyonthePCUtodetecttheislandingcondition.Instead,alargereactance,usuallyacapacitorbank,isinstalledontheutilitysysteminsidethepotentialislandatpointbasshowninFigure10.Theswitchisnormallyopen.Whentherecloseropens,thecapacitorbankswitchclosesafterashortdelay.InthecasewhereDPandDQwerenearlyzero,thesuddenadditionofalargereactiveimpedancewillimbalancethereactivepowerrequirement,causingafrequencydecreasewhichtheUFRcandetect.Thismethodothersseveraladvantages.Itishighlyeffectiveinpreventingislanding1,2aslongasthesmalldelayisallowedbetweenthetimeofrecloseropeningandthetimeofcapacitorinsertion.ThisdelayisnecessarytoensurethatinsertionofthecapacitorwillnotactuallycreateabalancedsituationbetweenthePVsystemandalaggingload.Capacitorsofthistypearereadilyavailable,andutilitieshaveagreatdealofexperiencewiththem.Thesamecapacitorbankcouldalsobeusedforreactivepowersupport,withonlyminorchangesintheswitchinglogictoallowittomaintainitsunit-islandingfunction.However,tworeadily-apparentdrawbackstothereactanceinsertionmethodare:
(1)theremaybemultipleswitchesinseriesleadingintothepotentialisland,meaningthateachseriesswitchmightneedtobeequippedwithaswitchablecapacitorbank(dependingontheloadconfiguration);
(2)thismethodcannotpreventshort-termislanding,partiallybecauseofthespeedofactionofthecapacitorswitchesandpartiallybecauseofthenecessarydelayinswitching.Thismethodactuallyhasathirdproblemwhichismorepoliticalthantechnical:
itrequirestheinstallationofequipmentontheutilitysideofthepointofcommoncoupling,whichisusuallytakentobetheutility'
selectricmeter.Utilitiesgenerallylookunfavourablyonsuchanarrangement.
METHODSOFELIMINATINGTHENDZ
PassivemethodsforislandingpreventioninvolvemonitoringthePCU'
sterminalvoltageforsomeconditionwhichindicatesislanding.Thesemethodsarediscussedbelow.
Thevoltageharmonicmonitoringmethoddoesnotrelyonarealorreactivepowermismatchforislandingdetection.Instead,thePVPCUmonitorsthetotalharmonicdistortion(THD)ofvaandshutsdownifthisTHDexceedssomethreshold.Therearetwomechanismswhichcancausetheharmonicsinvatoincreasewhenislandingbegins.OneoftheseisthePVPCUitself.APVPCUwillproducesomecurrentharmonicsinitsACoutputcurrent,asallswitchingpowerconvertersdo.Atypicalrequirementforagrid-connectedPVPCUisthatitproducenomorethan5%THDofitsfullratedcurrent.Summingatnodea,weseethatunderthisconditiontheharmoniccurrentsproducedbythePCUwillflowoutintothegrid.Whentherecloseropens,theharmoniccurrentsproducedbythePCUwillflowintotheload,andbyOhm'
slawandsuperposition,thesewillproduceharmonicsinva.18ThesevoltageharmonicscanbedetectedbythePCU,whichcanthenassumethatthePVsystemisislandinganddiscontinueoperation.
Iftheswitchwhichdisconnectstheutilityvoltagesourcefromtheislandisontheprimarysideofthetransformer,thesecondaryofthetransformerwillbeexcitedbytheoutputcurrentofthePVsystem.However,becauseofthemagnetichysteresisofthetransformer,thevoltageresponseofthetransformertothis(approximately)sinusoidalexcitationishighlydistorted.Inparticular,itcontainsalargethirdharmoniccomponent.Thisphenomenonhasbeenverifiedexperimentally;
investigatorshavefoundthatthethirdharmonicvoltageatnodeacanincreasebyafactoroffiveormorewhenislandingbeginsbecauseofthedistortionintroducedbystandardpole-mountedtransformers.
Intheory,thevoltageharmonicmonitoringmethodpromisestobehighlysuccess
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